The present invention relates to a micromechanical cap structure and a corresponding manufacturing method, the cap structure having a substrate, in particular in the form of a wafer, having a cavity made therein, the cavity having a bottom surface and two pairs of opposite parallel sidewall sections.
Although it can be used in principle for any micromechanical component, the present invention and the underlying principle are elucidated in greater detail with reference to this known surface-mounted micromechanical rotational speed sensor.
FIG. 4 schematically shows a cross-section through a known micromechanical rotational speed sensor which has a conventional micromechanical cap structure.
In FIG. 4, reference symbol 1 denotes a silicon substrate, 2 denotes a lower oxide layer, 3 denotes a buried circuit board made of polysilicon, 4 denotes an upper oxide layer, 6 denotes a bond frame made of epitactical polysilicon, 7 denotes a bond pad made of aluminum, 9 denotes a seal glass, 10 denotes a silicon protective cap wafer having a cavity K, 100 denotes a silicon wafer, 20 denotes an oscillator, 30 denotes a comb structure, VS denotes a front side, and RS denotes a back side.
According to conventional technology, micromechanical structures, in particular on front side VS, are exposed from underneath the 10 xcexcm thick polysilicon layer 6 by trenching and removal of the sacrificial layer (oxide 2, 4) underneath it. On the back side, silicon wafer 1 is subjected to deep etching.
Silicon wafer 1 is bonded to silicon protective cap wafer 10 through seal glass bonding at high-temperature and high pressure, the seal glass, i.e., glass solder 9, being applied on the cap wafer by the screen-printing method and subsequently sintered in a furnace process (prebake process).
Cavity K in silicon protective cap wafer 10 is produced by anisotropic micromechanical etching, for example, by a KOH wet etching method. Cavity K has an inverted truncated pyramid shape with (111) side surfaces and a (100) etching bottom.
For high-quality surface-mounted micromechanical rotational speed sensors, having low damping of the mechanical oscillator structure by media surrounded by gas, a low absolute pressure, i.e., vacuum encapsulation, is required.
A seal glass process has been developed in which gas encapsulation at less than 5 mbar is possible. This was achieved, among other things, by a largest possible volume of the cavity. It is furthermore advantageous to position the seal glass as far as possible from the inner edge of the cavity, which can be accomplished by outward-displaced screen printing.
However, narrow seal glass bonding, due to the outward-displaced screen printing, and a thin cap diaphragm with a high cavity volume reduce the stability of the capped chips for packing the mold, because a high pressure is produced during gluing onto the lead frame and during subsequent cooling due to the high hydrostatic pressure when pressing in the mold compound.
The micromechanical cap structure according to the present invention have the advantage compared to the known structure and the known method that increased stability of the bonded surface-mounted micromechanical component is possible, while good vacuum encapsulation and virtually unchanged outer chip dimensions are preserved.
The present invention provides reinforcement of the cap diaphragm without considerable volume limitations for the cavity and, as an alternative or additionally, provides a larger bonding surface at the chip corners without increasing the outer chip dimensions. This increases the stability of the sensor chip carrying such a cap for encapsulation in plastic mold housings.
The underlying idea of the present invention is that the cavity has at least one stabilizing wall section, which connects two side wall sections. The stabilizing wall section(s) may be located along the peripheral contour of the cap or may run on the bottom surface, thus dividing the cavity in a plurality of subcavities.
According to one preferred refinement, the cavity has a rectangular shape. Such a structure is easy to implement.
According to another preferred refinement, the cavity has two crossing stabilizing wall sections which run parallel to the side wall sections over the bottom surface. Such a supporting cross is very effective for stabilizing and is easy to manufacture.
According to another preferred refinement, the stabilizing wall section(s) are lowered with respect to the peripheral contour of the cavity. This has the advantage that no substantial space is lost in the cavity.
According to another preferred refinement, the cavity has two pairs of opposite parallel stabilizing wall sections along the peripheral contour of the cavity, which connect adjacent side wall sections so that the cavity has an octagonal shape, the internal angles preferably being equal to 135xc2x0.
According to another preferred refinement, the substrate is a silicon substrate, the bottom surface is a plane and the side wall sections are planes.